1. Field of the Invention
The present invention relates to an improvement to a thermal head which is used in a facsimile apparatus, a heat transfer printer, etc.
2. Description of the Related Art
A conventional thermal head has a structure in which an insulation substrate of alumina ceramics, for instance, seats a number of heating elements which are arranged as straight lines at a line density of 100 to 1,200 dpi (dot per inch) and a plurality of driver ICs (integrated circuits) which are electrically connected to the heating elements through a conductive layer or the like.
The driver ICs aim at selectively controlling heat generation in the plurality of heating elements which are respectively connected to the driver ICs in accordance with a printing signal which is supplied from outside, and therefore, a logic circuit such as a switching transistor, a latch and a shift register is formed inside each driver IC, and the same number of output terminals as the number of the heating elements whose heat generation is to be controlled by each IC are disposed to a mounting surface of each IC. The output terminals are electrically connected to the heating elements through the conductive layer or the like which is formed in a head substrate, so that each heating element develops Joule's heat when provided with an output current from the corresponding output terminal.
In such a thermal head, the heating elements selectively generate Joule's heat as driven by the driver ICs, the generated heat is transferred to a thermal recording medium such as a heat sensitive paper, and a printing image which corresponds to the printing signal is formed on the thermal recording medium.
A number of the heating elements are patterned by a sputtering method, a photolithographic technique or the like, in general, in such a manner that the respective heating elements have the same width and the same length with each other. Further, electric resistance values of the heating elements are adjusted at a high accuracy by a laser trimming method, a pulse trimming method or the like, and therefore, it is possible to ensure that the electric resistance values of the heating elements are exactly the same with each other.
However, in such a conventional thermal head described above, due to variations among the driver ICs caused during fabrication, internal resistance values which are inherent to switching transistors which are formed inside the driver ICs are somewhat different from each other among the driver ICs. Because of this, when generation of heat at a number of the heating elements is controlled using the plurality of driver ICs, an electric loss corresponding to the internal resistance of the switching transistor is created within each driver IC, so that the quantity of Joule's heat varies among the heating elements. As a result, a printing density varies among the plurality of heating elements which are connected to the driver ICs. Inconsistencies in density become noticeable particularly in an image such as a photograph of which is required to be finely expressed.